PARTIAL CHARACTERIZATION OF NOCARDIA-FARCINICA BETA-LACTAMASES

The beta-lactamases obtained from culture supernatants and cell extrac ts of 26 clinical strains and 5 reference strains of Nocardia farcinic a were partially characterized. The enzymes exhibited two patterns on isoelectric focusing (IEF). Beta-lactamases from the majority of the 3 1 strains (87%) including the 5 reference strains exhibited two major bands with pIs of 4.56 and 4.49. The remaining strains had two similar major bands but with slightly higher pIs. Culture supernatants and ce ll extracts exhibited identical patterns. The two sets of enzymes were functionally indistinguishable by substrate and inhibitor profiles an d lack of inducibility. By disk testing, ampicillin, amoxicillin, tica rcillin, amoxicillin-clavulanic acid, and imipenem were highly synergi stic with cefotaxime. The enzymes were primarily penicillinases and hy drolyzed cephalosporins at rates of less-than-or-equal-to 12% of those for penicillins. N. farcinica beta-lactamases were susceptible to inh ibition by clavulanic acid and BRL 42715, exhibiting 50% inhibitory co ncentrations of 0.025 to 0.045 mug/ml (0.12 to 0.22 muM) and 0.05 to 0 .1 mug/ml (0.31 to 0.63 muM), respectively, less susceptible to tazoba ctam, and least susceptible to sulbactam, cloxacillin, and imipenem. T he beta-lactamases of N. farcinica are believed to mediate penicillin resistance and may play a secondary role in extended-spectrum cephalos porin resistance. The close similarity among N. farcinica beta-lactama ses and their distinct differences from beta-lactamases of other Nocar dia species support the taxonomic identity of this species.